Considerable effort has been made to develop imaging compositions, particularly those useful in the graphic arts, which do not require the use of silver. The reason, of course, is the high cost of silver. One such nonsilver system involves the use of cobalt(III) complexes which are reduced as a result of a photolytic reaction to form as reaction products cobalt(II) and a released ligand, such as ammonia. Either of these can in turn be used to form images, the cobalt(II) being chelated by compounds capable of forming tridentate chelates, or the ammonia or other ligand being reacted with dye precursors, including diazo-coupler systems, to form a dye; or they can be used to bleach out preincorporated dye. Such compositions are disclosed in Research Disclosure, Vol 126, October, 1974, Publication No. 12617, Part III, published by Industrial Opportunities Limited, Homewell, Havant Hampshire P091EF, United Kingdom. Amplification can be achieved by using a reducing agent precursor capable of producing internal gain by forming with the aforesaid reduction products a reducing agent for the reduction of remaining cobalt(III) complexes. For example, certain of said chelating compounds for cobalt(II) form when chelated a reducing agent, as described in Research Disclosure, Vol 135, July, 1975, Publication No 13505. Alternatively, o-phthalaldehyde will react with ammonia to form a reducing agent, as disclosed in Research Disclosure, Vol 158, June, 1977, Publication No 15874.
All such compositions feature the release of ligands, preferably amines. Quite often, however, the ligand release, particularly when amplified by the mechanisms noted above, has been discovered so effective in forming images that excessive contrast can occur. As a result, these compositions often exhibit very short processing latitude over time or temperature, or short exposure latitude such as when reproducing halftone dots. Although these processing and exposure latitude characteristics are useful in certain cases, particularly when photographing line copy, they can be undesirable in other applications, such as in the reproduction of continuous tone images, where extended processing and exposure latitude are advantageous.
Therefore, there has been a need to modify the cobalt(III) complex imaging compositions in a manner that will provide an imaging composition and element having improved photographic properties.
It has been known that halogenated methyl-s-triazines will react with ammonia, as noted by Schaeffer and Ross, "Chlorination and Bromination of Alkyl-s-Triazines", J Organic Chemistry, Vol 29, page 1527 (1964). However, there is no suggestion in this article that such a reaction can control photographic properties, or indeed that this reaction has any relation to recognized image-forming chemistry.